Traditionally, vehicles which operate over adverse terrain have employed track drive systems. Track driven bulldozers, loaders, excavators, cranes and other heavy industrial vehicles have been in use for decades, as have armored tanks and other track driven military vehicles. The tracks are comprised of rigid tread elements pivotally interconnected in belts supported for movement around courses on the sides of the vehicle. Propulsion is effected by simultaneous drive of the tracks, and the vehicle is steered by differential drive thereof.
The capability of operating over rough ground is the primary advantage of a track driven system. By nature, however, vehicles incorporating such drive systems cause at least some surface damage during operation, and particularly while maneuvering. Surface damage is not a major problem as far as operation over open ground, but it can be a serious drawback with respect to operation over smooth or paved surfaces due to the resultant permanent damage thereto. Consequently, track driven vehicles have been restricted to operation over rough or unprepared surfaces as a practical matter.
More recently, wheeled vehicles have been developed which are capable of operation over smooth and rough surfaces without causing such surface damage. U.S. Pat. No. 3,799,362, entitled "Six Wheel Material Handling Vehicle" and assigned to the assignee hereof, illustrates a successful example of one such vehicle. Other vehicles incorporating multiple wheels and skid steering principles have been developed especially for use with particular mechanisms.
In addition, multiple wheeled undercarriage assemblies utilizing lowered middle wheels to facilitate skid steering have been developed heretofor for supporting and propelling some mechanisms. With larger and thus heavier mechanisms, however, the advantages of such undercarriage assemblies have been offset to a considerble extent by reduced maneuverability and turning ability stemming from excessive ground pressure beneath the wheels. Excessive ground pressure in turn causes ruts and other surface damage. A need has thus arisen for a multi-wheeled undercarriage unit for supporting and propelling a relatively heavy mechanism over smooth or adverse terrain without substantial losses in maneuverability or turning ability and within acceptable ground pressure levels.